While optical recording, i.e. the recording and playback of information using a focused beam of light, was first proposed long ago, it required the development of low cost, practical lasers to make such a concept a commercial reality. Today, vast sums have been and are being invested in research and development aimed at developing more sensitive media, media having higher resolution, systems having improved capabilities of coping with physical irregularities, etc. A useful review of what may reflect the state-of-the-art in many competing media for optical recording is presented in the article "Optical Disk Systems Emerge", by R. A. Bartolini et al, IEEE Spectrum, Vol. 15, No. 8, pages 20-28, August 1978, at page 22, of which is set forth a table comparing various candidate materials and media. A particularly desirable medium which bears certain similarities to the present invention is discussed at page 26 thereof. The medium there discussed includes a thin layer of metal such as titanium, which upon impact of a laser beam, melts (the melting point at Ti is 1668.degree. C.) to form a crater pit; thus allowing a reflectorized layer therebelow to become visible.
Optical recording media utilizing thin metallic films which form pits upon recording are further discussed in the article "Optical Recording with the Encapsulated Titanium Trilayer", by A. E. Bell et al, RCA Review, Vol. 40, pages 345-362, September, 1979. That article notes that dust particles tend to degrade the media after fabrication, and proposes that the media be encapsulated with a silicone rubber formulation (such as G.E. RTV 615B) to protect the surface. However, due to the high temperature present at which the Ti layer melts to form the crater pits, it is further said to be necessary to thermally isolate the protective overcoat by providing a silicon dioxide thermal barrier layer between the Ti absorber layer, and the overcoat. At page 359 the authors report on recordings made after deposition of the silicon dioxide overcoat thermal barrier layer, but prior to application of the RTV coating, and observed that "some distortion of the silicon dioxide layer has occurred, resulting in the formation of raised bubbles in the regions where the titanium layer has been melted."
A further mention of the formation of domes in the process of melting a thin metallic film upon laser impingement to form holes for digital data storage is found in the article "Melting Holes in Metal Films for Real-Time, High-Density, Permanent Digital Data Storage", by Messrs. John Corcoran and Herman Ferrier, Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 123, pages 17-31. At page 19 thereof are reported results obtained upon laser impingement on experimental media constructed of a substrate of glass on which an 80 nm thin film of chromium was deposited and on top of which was a layer of collodion (a viscous solution of pyroxylin, a cellulose nitrate mixture). When a laser beam limited in intensity to a point where resultant holes could not be detected optically was directed onto the Cr film, dimples in the outer surface were detected upon SEM examination. When the collodion layer was stripped off and the Cr layer alone exposed as before, domes having a radius of approximately one micrometer were detected via scanning electron microscopy. The authors postulated that both the dimples and domes resulted from gaseous decomposition products formed when the Cr film melted, and which, given slightly more energy, would produce holes.
The use of thin metal or metal-like films in direct-read-after-write recording media in which the films are caused to melt or ablate to form holes or pits likewise is disclosed in numerous patents, none of which, however, contains any suggestion as to the formation of bubbles, domes, protuberances, or the like, on the surface of the media. See, for example, U.S. Pat. Nos. 3,560,994 (Wolff & Hanisch), 3,720,784 (Maydan et al), 3,787,873 (Sato), 3,889,272 (Lou), 3,911,444 (Lou), 4,000,492 (Willens), 4,023,185 (Bloom et al), 4,069,487 (Kasai et al), 4,097,895 (Spong), 4,101,907 (Bell and Bartolini), 4,137,077 (Credelle et al), 4,139,853 (Ghekiere et al), 4,141,731 (Jarsen), 4,176,377 (Howe), 4,188,214 (Kido et al), 4,189,735 (Bell et al), 4,195,312 (Bell et al), 4,195,313 (Bell et al.), 4,211,617 (Hunyar), 4,216,501 (Bell), and U.K. Pat. No. 1,571,948 (Thomson-Brandt).
These patents are primarily directed to media which include light-absorbing films of low melting point metals such as Bi, Pb and Sn, and those of moderate melting points metals such as Ti, but in which refractory materials would normally be undesirable. This undesirability is evident in that the amount of energy required to melt the material and to form the pits or craters, i.e. the recording threshold, would be excessively high, thus resulting in a recording medium with a lower equivalent sensitivity, assuming that all other parameters in the media were the same, i.e. that the respective media likewise include or do not include such sensitivity affecting elements as light reflecting underlayers, antireflecting overlayers, interferometric dimensions to optimize absorption and/or reflection, etc.
Of the above listed patents, U.S. Pat. No. 4,069,487 (Kasai et al) is particularly relevant to the present invention, as it suggests that the recording layer in which pits or holes are to be formed by vaporization and/or melting is composed of a non-metallic material which absorbs the laser beam efficiently. Representative non-metallic materials are said to include inorganic oxides, chalcogen compounds and resins, relatively high sensitivity materials being suggested to include lead oxide, tungsten oxide, titanium oxide, silicon oxide, zirconium oxide, and the like. The melting points of these oxides are believed to be approximately 1160 K., 1740 K., 2110 K., 1980 K. and 2970 K., respectively, such that some of them may be said to be a refractory material as defined below. Such oxides are said to be usable alone as the light-absorbing layer, as in Example 4 thereof, wherein a 300 nm layer of WO.sub.3 was deposited, as well as when laminated with a metallic layer, as in Example 4, wherein a 60 nm thick layer of TiO.sub.2 was deposited on top of 100 nm thick layer of Au. The pertinent figures of that reference depict holes or pits formed during recording as extending through such laminate constructions to expose the support therebelow. The patent thus totally fails to suggest formation of protuberances during recording.
Optical recording media which include oxygen deficient oxides of Ti, Fe, Cr, Mn, Pb, and Zr are disclosed in U.K. Pat. No. 1,571,948, wherein recording under a laser beam is said to produce localized changes in the index of refraction. While certain of those oxides may likewise be considered to be refractory, the patent contains no suggestion of the formation of protuberances.